A selective fire weapon doesn't necessarily "require" more ammunition, it just gives you the option of a high rate of fire. What you need depends on the situation. A bolt action sniper rifle would suck for close quarters combat, especially against multiple assailants.

"I am attacking an idea which I believe to be false; a system which appears to me to be unjust; an injustice so independent of personal intentions that each of us profits from it without wishing to do so, and suffers from it without knowing the cause of our suffering." - Frédéric Bastiat

Or you could always go with a weapon which is capable of rapid fire semiauto and accurate enough for sniper work. There are a number of springfield M1A variants which are more than adequate. A good national match M1A is a very hard weapon to beat in all around versatility for someone who intends to do more long work than short, and a short barrel socom or scout sniper M1A allows for a smaller footprint while still allowing accuracy for the long shot. And the .308 or 7.62x51 is a tough round to beat for impact. Its probably good for everything in North America short of a Grizzly, and might even take one of those with a properly placed shot.

Or you could always go with a weapon which is capable of rapid fire semiauto and accurate enough for sniper work. There are a number of springfield M1A variants which are more than adequate. A good national match M1A is a very hard weapon to beat in all around versatility for someone who intends to do more long work than short, and a short barrel socom or scout sniper M1A allows for a smaller footprint while still allowing accuracy for the long shot. And the .308 or 7.62x51 is a tough round to beat for impact. Its probably good for everything in North America short of a Grizzly, and might even take one of those with a properly placed shot.

I wonder if there are any advantages of an automatic rifle over a sniper rifle.

A sniper rifle can be used for hunting, requires much less ammunition, and seems more suitable to be used defensively.

There are some good custom built semis that can be used for both. I believe River Rock makes a AR for varmint hunting that is pretty kick ass from what I have read.
See thru mounts would be the way to go for the compromise to get the best of both worlds. But don't skimp on your scope though for it would have to be quality built to handle extreme conditions. I would guess for a set up like the above your looking at no less than 4 or 5 grand with a night vision scope. But I do want to say,do not skimp on your scope or your gun as a sniper rifle will be useless pretty much.

Or you could always go with a weapon which is capable of rapid fire semiauto and accurate enough for sniper work. There are a number of springfield M1A variants which are more than adequate. A good national match M1A is a very hard weapon to beat in all around versatility for someone who intends to do more long work than short, and a short barrel socom or scout sniper M1A allows for a smaller footprint while still allowing accuracy for the long shot. And the .308 or 7.62x51 is a tough round to beat for impact. Its probably good for everything in North America short of a Grizzly, and might even take one of those with a properly placed shot.